In ear headphones are spatially limited by the size of the user's external auditory canal and pima. Given such limitations, monitoring sensors are necessarily space limited as well. At the same time, however, such systems are required for proper device function. One such example is a device requiring at least one LED light source. Emissions of the LED introduce varying levels of light spillage. Such levels of light spillage in significantly confined structures are problematic by introducing unacceptably high levels of artifact. These artifacts may produce false negative or false positive results. Various methods have been suggested to limit this side spillage of the generated light. These include the use of expensive LEDs with coned apertures. Such apertures limit the effective amount of side spillage. These have the disadvantage of much greater size and cost. Another method of controlling side spillage is through the use of baffles or shields. These systems have the disadvantage of increasing both the size and the bulk of the device. What is needed is a new way to prevent light spillage from LED light sources.